This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. Primary support for the subproject and the subproject's principal investigator may have been provided by other sources, including other NIH sources. The Total Cost listed for the subproject likely represents the estimated amount of Center infrastructure utilized by the subproject, not direct funding provided by the NCRR grant to the subproject or subproject staff. Anti-cancer therapy has typically been targeted on neoplastic cells. Inhibitors of the enzyme thymidylate synthase (TS) have been used for many years in the clinical management of a variety of cancers. In spite of the extensive research on the genetic and molecular factors governing tumor response to TS inhibitors, its clinical efficacy remains limited. In this project, we propose a novel approach to increase tumor response to these agents. Tumors are infiltrated with a heterogeneous population of non-neoplastic cells. These include host-derived cells such as fibroblasts, macrophages, lymphocytes, endothelial cells, etc. Together with extracellular matrix, they make up the tumor stroma or microenvironment. By secreting an array of cytokines, growth factors, hormones, etc., they play a critical role in tumor growth and progression, and response to therapy. We will test the hypothesis that tumor response to TS inhibitors is governed by the chemosensitivity of infiltrating stromal cells. We utilized the ApcMin/+ mice which spontaneously develop adenomatous polyps in the small intestine and the colon. By bone marrow transplantation we generated chimeric mice wherein the chemosensitivity of stromal cells is distinct from that of the tumor and predicted that tumors in these mice will show a drug response that reflects the chemosensitivity of stromal cells. The specific aims are: 1) to determine the impact of TS inhibitors on cells in the stromal compartment and identify mediators of response to TS inhibitors;2) to examine the effect of TS down regulation in stromal cells on tumor response to TS inhibitors;and 3) to direct sensitization to TS inhibitors specifically to tumor associated stromal cells. The hypothesis being tested will pave the way for development of new treatment strategies using stromal cells to improve therapies targeted at tumor cells to ensure drug induced cancer cell death.